Abstract
We introduce a novel method for correcting distortion in thin silicon substrates caused by coating stress. Thin substrates, such as lightweight mirrors for x-ray or optical imaging, and semiconductor wafers or flat panel substrates, are easily distorted by stress in thin film coatings. We report a new method for correcting stress-induced distortion in flat silicon substrates which utilizes a micro-patterned silicon oxide layer on the back side of the substrate. Due to the excellent lithographic precision of the patterning process, we demonstrate stress compensation control to a precision of ~0.2%. The proposed process is simple and inexpensive due to the relatively large pattern features on the photomask. The correction process has been tested on flat silicon wafers that were distorted by 30 nm-thick compressively-stressed coatings of chromium, achieving RMS surface height and slope error reductions of a factor of 68 and 50, respectively.
Highlights
The National Aeronautics and Space Administration (NASA) Lynx x-ray telescope mission concept is currently under study for consideration by the 2020 Decadal Review [1]
At the NASA Marshall Space Flight Center (MSFC), monitored stress in situ during deposition onto 50 mm-diameter flat silicon wafers, and was able to reduce the stress in 15 nm-thick iridium films to ~3 MPa (~0.05 N/m of integrated stress, i.e., the stress in the film integrated over its thickness, which is equivalent to the mean film stress multiplied by the film thickness) [10], which may meet Lynx requirements [11]
We demonstrate a process for stress compensation of 100 mm diameter, 525 μm thick flat silicon wafers distorted by 30 nm thick chromium coatings
Summary
The NASA Lynx x-ray telescope mission concept is currently under study for consideration by the 2020 Decadal Review [1]. A group at the NASA Goddard Space Flight Center (GSFC) has developed a process for producing thin silicon mirrors with high angular resolution (~1”) using a relatively simple, low cost process [5] These mirrors have come close to meeting requirements for Lynx, and rapid progress continues to be made. They attempted to balance the compressive stress in the iridium film by depositing chromium film under tensile stress beneath the iridium, or by using atomic layer deposition to coat iridium films on both sides of the mirror, but stress nonuniformity resulted in a poor balance and a 1-2 μm mirror sag error which is far beyond the Lynx requirement [9] Another group, at the NASA Marshall Space Flight Center (MSFC), monitored stress in situ during deposition onto 50 mm-diameter flat silicon wafers, and was able to reduce the stress in 15 nm-thick iridium films to ~3 MPa (~0.05 N/m of integrated stress, i.e., the stress in the film integrated over its thickness, which is equivalent to the mean film stress multiplied by the film thickness) [10], which may meet Lynx requirements [11]. Based on the fact that thermal oxide grown on silicon substrates generates repeatable ~-300 MPa (compressive) stress [16,17], which is extremely stable, we developed a process to produce thermal oxide patterns on the backside of mirrors to compensate for coating stress
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